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Monday, June 30, 2008

Carbon Nanotubes and Cancer: A Hazard Forestalled?

Writing about engineering ethics can be a grim business: plane crashes, fires, explosions, or insidious deaths from apparently innocuous things that people did decades earlier show up with depressing regularity in this column. So I try to look for good news every now and then, and a few weeks ago some showed up in the New York Times, of all places. The good news is not unmixed with bad news, but hey, I'll take what I can get.

The bad news is that it looks like carbon nanotubes—those tiny rods of carbon atoms arranged on a hexagonal grid like chicken wire—may have the potential to cause cancer under certain special conditions. The good news is, we know about the problem long before any epidemiological evidence has shown up that people have actually been harmed in this way. This looks like a success story of how research dollars spent on engineering-ethics-related matters have actually paid off with some useful information. But first, let me describe carbon nanotubes and summarize the results of the research.

Starting in the early 1990s, many scientists began to be aware that under certain conditions, carbon forms long, thin tubes that can be less than a billionth of a meter in diameter but thousands of times longer than that. The term "nanotube" was invented to describe these structures, and since then there has been a race of sorts both on the scientific and engineering fronts to exploit their fascinating properties. One peculiarity they show is that they conduct electricity extremely well along the axis of the tube, so well that I understand one prominent commercial application is in carbon motor brushes. Anybody who uses a power-line-powered electric drill has used carbon brushes—sometimes you can see sparks from them toward the back of the drill. The purpose of the brush is to conduct current to the armature (the moving part) of the motor. Brushes made with properly aligned nanotubes are more efficient at this than the regular kind, although their added cost doesn't justify using them in consumer products such as drills. But in heavy industrial applications where the brushes may conduct hundreds of amps, they make enough difference to sell.

Anyway, some researchers in the United Kingdom and the U. S. got some government funding to see whether carbon nanotubes might be dangerous to health. One reason to suspect this might be the case has to do with the shape of the nanotubes: long and thin. It turns out that one of the main reasons asbestos particles can cause an otherwise rare form of lung cancer called mesothelioma appears to be that they, too, are long and thin (that's why asbestos is a mineral fiber that can be woven into sheets). Although the connection between the shape of asbestos fibers and this type of cancer is not entirely clear, there is abundant evidence that associates exposure to asbestos with mesothelioma. So the researchers thought it would be worth looking into to see if carbon nanotubes could cause precancerous lesions in mice.

Well, they do. The experiment wasn't continued to the point that the mice actually developed cancer, but they did get inflammation and certain lesions associated with precancerous conditions.

The first thing that will happen is, the researchers will apply for more funding to look into the question further. That seems reasonable, because it's a long stretch from precancerous lesions in mice to actual cancer in humans. The next thing that ought to happen is that people should use appropriate precautions when dealing with carbon nanotubes. What would "appropriate precautions" amount to?

Well, nothing like an outright ban, for example. We have learned how to deal with all kinds of hazardous substances over time, and carbon nanotubes don't seem to be nearly as hazardous as some other kinds of stuff you have around the house—drain cleaner or bleach, for example. If you can keep people from breathing or swallowing carbon nanotubes, the things should be perfectly safe to use otherwise. This may present a problem if someone wants to weave them into clothing, for example, but most of the interesting applications have nothing to do with textiles.

On the other hand, having been forewarned by at least one forward-looking study, we shouldn't totally ignore the potential health hazards that carbon nanotubes might present in the future. All too often, the typical way that health hazards of newly introduced substances have been discovered is that people simply start selling products with the new stuff in them, and months or years later, some weird rare malady starts showing up in a few people (or sometimes not so few). And it takes a lot of epidemiologists doing a lot of Sherlock Holmesing to find the cause. By the time they do, whole industries dependent on the substance in question may have large vested interests in the status quo, and so you get a big political tussle as well as delays in appropriate regulations, if they are enacted at all.

This happened with asbestos and, above all, tobacco, and I hope it doesn't happen with carbon nanotubes. Fortunately, it looks like you can't smoke them, so we probably have less to worry about than with cigarettes. Nevertheless, I hope this study doesn't just disappear and fail to stimulate more research into the possible health hazards of carbon nanotubes. This time, maybe, we can get it right.

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Ethical and Otherwise

Ethical and Otherwise:Engineering In the Headlines is a collection of over 40 of the most popular blog posts selected from nearly ten years of Karl Stephan’s engineering ethics blog.Bridge collapses, airplane crashes, train wrecks, explosions—the most widely read blog posts are all here, from the truth about why the Titanic sank to the latest recall scandal.This ebook is available in both Kindle and iBook formats.

Analog and Mixed-Signal Electronics

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A practical guide to analog and mixed-signal electronics.Covers elementary analog circuits, analog-to-digital and digital-to-analog circuits, high-frequency circuit design, and power electronics.Recommended for EE undergraduates, advanced students, and for those involved in computer engineering, biomedical engineering, computer science, and physics.Publisher:Wiley (2015). See this book on Amazon.